Transmission-electron-microscope (TEM) analysis of site-specific lamellae is becoming increasingly advantageous in early process development of microelectronic devices—particularly for failure analysis of device components. By “site-specific lamella” is meant an electron-transparent membrane having within the membrane a specific region that is to be examined under an electron microscope. Conventional methods of preparing site-specific lamellae (particularly in a plan-view orientation) from multilayered microelectronic devices are unfortunately deficient in many respects, including that these methods: (a) generally are capable of producing acceptably-electron-transparent lamellae of only relatively limited area dimensions (e.g., for reasons later herein detailed); and (b) often present challenges for end-pointing {i.e., identifying (or “pointing” to) which layer of a multilayered microelectronic device is the target layer containing the region of interest to which the process of edge-on FIB-based thinning should progress—while, on exposure of that target layer, the process of edge-on FIB-based thinning should cease (or “end”)}.
More specifically, conventional methods of preparing site-specific, plan-view lamellae for TEM analysis often preclude process development engineers from, at least on a routine basis: (a) obtaining high quality lamellae (e.g., lamellae that are both largely free of analysis-interfering artifacts and have more-than-adequate electron transparency over sufficiently large areas); and (b) implementing processes amenable to straightforward end-pointing on exposure of, among layers of a multilayered microelectronic device, the target layer of interest—this latter preclusion also prevents process engineers from putting in place procedures for robust automation in end-pointing.
Failure analysis of a microelectronic device often involves failure analysis and fault isolation in a component integrated circuit (IC) device. In general, when the cause of a failure in an IC device is expected as originating from a defect in the metallization stack on the top of a device substrate, preparation of cross-sectional TEM specimens may often be identified as being particularly helpful for failure analyses. However, when the causative fault is indicated (e.g., from an electrical signature of a failure) as lying in the substrate (e.g., in dislocations or some other crystal defect), preparation of plan-view TEM specimens may often be seen as being particularly helpful for failure analyses. (1) R. Anderson and S. J. Klepies, Practical aspects of FIB specimen preparation, with emphasis on semiconductor applications, In: L. A. Giannuzzi & F. A. Stevie (ed's) Introduction to Focused Ion Beams—Instrumentation, Theory, Techniques and Practice. Springer Science, pp. 173-200, 192-193 (2005).
Regardless of the motivation for preparing a TEM specimen, however, a relatively straightforward, conventional “lift-out” method (either through ex-situ or in-situ means) is typically used to prepare a cross-sectional TEM specimen. In such lift-out method, a cross-sectional slice of material is lifted out of a work piece after the work piece has been milled perpendicularly to its surface, i.e., after cross-sectional cuts corresponding, respectively, to each side of the lift-out slice are milled through the work piece perpendicularly to the layers—with typically at least one cut, or often both cuts, being made in the form of a trench, e.g., possibly using a “stair step” milling algorithm—after a 0.5 to 1 μm thick metal line of platinum (Pt) or tungsten (W) has been deposited on the upper surface of the lift-out slice to protect it from ion beam damage. (2) L. A. Giannuzzi et al., FIB lift-out specimen preparation techniques: ex-situ and in-situ methods, In: L. A. Giannuzzi & F. A. Stevie (ed's) Introduction to Focused Ion Beams—Instrumentation, Theory, Techniques and Practice. Springer Science, pp. 201-228, 205-221 (2005). After being lifted out of the work piece, the slice is next fixed (e.g., typically yet in a vertical orientation) onto a TEM grid, where it may be further thinned (e.g., to at least an acceptable level of electron transparency) from either side, e.g., using additional FIB milling (again perpendicularly to the work piece's internal layers) or using low-voltage ion milling at a grazing incidence. See, e.g., FIGS. 2-4 and associated text in “Background of the Invention” section of (3) U.S. Pat. No. 7,423,263 (the '263 patent—issued Sep. 9, 2008) to Liang Hong et al., “Planar view sample preparation” (Assignee: FEI Company).
In a conventional method for preparing a plan-view lamella, in contrast, a wedge or chunk is first cut from work piece material by FIB milling. Standard gallium ion (Ga+) FIB milling is commonly used to cut the wedge in from the work piece, although such wedge-defining milling may proceed relatively slowly and require acceptance of some embedding in the wedge of contaminant Ga+ ions.
After this wedge is removed from the work piece, its orientation is tilted by 90°—possibly through rotating a carrier to which the wedge is mounted, e.g., so that previously horizontal wedge layers are now oriented vertically and potentially parallel to the ion beam column. See also FIGS. 10-12 and associated text of (3) U.S. Pat. No. 7,423,263; see also panels (b) to (d) of FIG. 3 of (4) J. Mayer, L. A. Giannuzzi, T. Kmino, and J. Michael, TEM sample preparation and FIB-induced damage, MRS [Materials Research Society] Bulletin, Vol. 32, pp. 400-407 (May 2007). The re-oriented wedge is then thinned, e.g., “bulk thinned” using FIB milling edge-on to the wedge's layers, to generate a plan-view lamella. Because the edge-on milling removes all material along the beam axis, the resultant lamella comprises a thinned section extending from the top of the lamella to the bottom of the lamella, with thicker borders on one or both sides.
Edge-on FIB milling alone, however, cannot solely be relied on to prepare a plan-view lamella consisting of a target layer containing the region of interest and having an acceptable level of planarity. A process of slow and repetitive, glancing-angle cleaning in an FIB system—accompanied by many FIB system adjustments (e.g., for scan rotation, z-dimension depth, ion column tilt, or pixel overlap)—is often required to remove artifacts or to address areas of varying hardness on a lamella. In addition, simply correctly identifying and selecting the target layer containing the region of interest (i.e., identifying the previously horizontal layer within a re-oriented wedge from which a plan-view lamella is to be prepared) is often challenging when using this conventional method for preparing a plan-view lamella. Considerable effort may be wasted on completing repetitive, glancing-angle cleaning of a presumed target layer only to learn subsequently that the cleaned layer had been misidentified and is in fact not the target layer.
How thinly a TEM specimen may be milled using a conventional method is largely limited by how closely the line of FIB milling on one side of the TEM specimen may realistically approach the line of FIB milling on the other side of the specimen—without the specimen (i.e., the nascent or intended lamella) voiding the specimen-preparation effort by bending, curling, shrinking, or otherwise warping. If a specimen that is being bulk-thinned through FIB milling does bend, curl, or otherwise warp during FIB milling, the ion beam most likely will destroy the specimen or render it essentially unusable for TEM analysis. Because edge-on thinning produces a thin center section that extends from the top of the lamella to the bottom of the lamella, the lamella lacks a thicker support along the top and bottom to resist warping.
Accordingly, in order for edge-on FIB milling of a specimen from a re-oriented wedge of a multilayered microelectronic device to generate a plan-view lamella that is sufficiently thin for successful TEM analysis (e.g., often a thickness of less than about 100 nm may be required for a lamella to have acceptable electron transparency), the proclivity of the specimen (particularly a larger-area specimen) to bend, curl, or otherwise warp while being thinned must be forestalled. See, regarding 100 nm thickness of lamella, (5) U.S. Pat. Appl. Pub. No. 20160126060 (the '060 patent application—published May 5, 2016) to Fuller et al., “Endpointing for focused ion beam processing” (Applicant for parent PCT Application: FEI Company) (“[0003] . . . While a SEM [scanning electron microscope] can observe a feature on a thick work piece, to observe a sample on a TEM, it needs to be thinned to less than 100 nm so that electrons will travel through the sample.”). As used herein, a sample or specimen is an object extracted from a work piece. A lamella is a thin, electron-transparent membrane form of a sample.
Although the difficulty of preventing such bending, curling, or warping will generally increase as a lamella becomes larger in area and thinner in depth, a lamella of a larger area is typically preferred for precise TEM analysis, and some TEM analysis may even require that a lamella be prepared having a thickness of less than about 20 nm. See (6) U.S. Pat. Appl. Pub. No. 20130328246 (the '246 patent application—published Dec. 12, 2013) to Wells et al., “Lamella creation method and device using fixed-angle beam and rotating sample stage” (Assignee: FEI Company) (“[0009] . . . Lamellae are typically less than 100 nm thick, but for some applications a lamella must be considerably thinner. With advanced semiconductor fabrication processes at 30 nm and below, a lamella needs to be less than 20 nm in thickness in order to avoid overlap among small scale structures.”).
The proclivity of a plan-view specimen prepared according to the above-summarized conventional method to bend, curl, or otherwise warp often increases to an unacceptable level when the TEM specimen's area of electron transparency (again, which typically has a thinness of less than about 100 nm) is roughly larger than about 5 μm×5 μm=about 25 μm2 (or, in some cases, simply larger than about 2 μm×2 μm=about 4 μm2). As a result, a plan-view lamella being prepared from a re-oriented wedge taken from a multilayered microelectronic device using the above-noted conventional method is typically limited to a size roughly less than 5 μm×5 μm=about 25 μm2 (and, in some cases, simply less than about 2 μm×2 μm=about 4 μm2).
In addition, preparation of a site-specific, plan-view lamella (e.g., one corresponding to a target layer of a multilayered microelectronic device) using the above-noted conventional method is also not amenable to straightforward end-pointing (nor to automation in end-pointing). This is so in part because, for example, identifying (or “pointing” to) which layer of a multilayered microelectronic device is the target layer containing the region of interest to which the process of edge-on FIB milling should progress—while, on exposure of that target layer, the process should cease (or “end”)—is often inordinately difficult when using the above-noted conventional method for plan-view lamella preparation.
For example, a key identifying characteristic of an individual layer—its planar-oriented, lithography-generated circuit pattern—may often need to be estimated by an expert only from cut circuit lines along the exposed side planes of a re-oriented wedge. But observing less-than-clear, edge-on views of some of the possibly 35 or more sliced layer edges of a microelectronic device may often lead an expert to identify a specific layer only tentatively. Nonetheless, an expert will very likely be stuck with relying on such less-than-clear, edge-on views of cut circuit lines because, for example, unobstructed views of planar-oriented circuit patterns of individual layers may not be available during most of an edge-on FIB milling process (i.e., the bulk thinning process).
Furthermore, even if edge-on FIB milling were fortuitously to progress to the correct target layer, subsequently identifying certain sections of the target layer's circuit pattern often presents difficulties. This is so in part because planar surfaces (often marginally planar) exposed through such edge-on FIB milling (even after repetitive, glancing-angle cleaning) often are, alternatively: (i) damaged by embedded Ga+ ions (e.g., as may result from an ion beam being directed at an angle that is not optimized for surface cleaning—possibly through a miscalculated compensation for an expected Gaussian shape of the ion beam); (ii) yet obstructed by curtaining artifacts (i.e., by striations or veils that may result, e.g., where materials having a low sputtering yield block faster sputtering yield materials); or (iii) characterized by an exposed circuit pattern that shares many commonalities with a circuit pattern of an adjacent layer (e.g., as is often the case for 3D-NAND memory devices)—so that unequivocally distinguishing sections of a target layer's circuit pattern from those of an adjacent layer may yet remain challenging.
At least for these above-noted reasons, it would be desirable to provide a FIB-based method for preparing a site-specific, plan-view lamella of a region of interest in a multilayered microelectronic device wherein the method both allows for the routine preparation of lamellae of an area larger than at least about 100 μm2 (e.g., larger than at least about 10 μm×10 μm) and is clearly amenable to straightforward end-pointing (including automation in end-pointing).